Add API-doc site, format gate, recipe.yaml, bench

[conorbronsdon]

Rolls the suite-wide quick wins proven on mojo-xml and mojo-feed onto this repo:

• Canonical format commit — one-time mojo format pass over src/, test/, examples/ (only examples/list_archive.mojo changed).
• Format gate in CI — test workflow now fails if sources aren't mojo format-clean; fix locally with pixi run fmt.
• API-doc site — docs/render_api.py (copied from mojo-xml) renders mojo doc JSON to a self-contained HTML page; docs.yaml workflow builds and deploys it to GitHub Pages; pixi run docs builds it locally.
• recipe.yaml — rattler-build recipe shipping the pure-Mojo source tree with a runtime mojo pin (same pattern as mojo-xml; not wired to CI).
• bench/ — bench/bench_read.mojo times the core read path (open_tar: header parse + member iteration) over the repo's own test/data/*.tar fixtures. Fixtures are tiny (10 KiB), so each is parsed 5000x per measurement; the bench only reads trusted in-repo fixtures and adds no parsing logic. Local pixi run bench only, not in CI.
• New pixi tasks: fmt, docs, bench; .gitignore covers generated docs and the bench binary.
• Docstring lint: package summary in src/tar/__init__.mojo now starts capitalized (mojo doc emits no summary warning).

Local verification: pixi run test 38/38 pass; pixi run docs writes docs/api.html; bench compiles and runs; pixi run fmt && git diff --exit-code clean.

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